The term “biomass” generally relates to any material that is derived from living, or recently living biological organisms. In the energy context it is often used to refer to plant material, however by-products and waste from livestock farming, food processing and preparation and domestic organic waste, can all form sources of biomass. Biomass is widely available and contains a high proportion of cellulose, hemicellulose and lignin. The four main categories of biomass are: (1) wood residues (including sawmill and paper mill discards), (2) municipal paper waste, (3) agricultural residues (including corn stover and corn cobs and sugarcane bagasse), and (4) dedicated energy crops (which are mostly composed of fast growing tall, woody grasses such as switch grass and Miscanthus). Lignocellulosic biomass contains three primary polymers that make up plant cell walls, namely, (1) cellulose, a polymer of D-glucose; (2) hemicellulose which contains two different polymers i.e. xylan, a polymer of xylose and glucomannan, a polymer of glucose and mannose; and (3) lignin, a polymer of guaiacyl propane and syringyl propane units. Of these components cellulose is believed to be most desirable since it can be converted into monomer glucose that can be fermented to ethanol.
The exemplary embodiments described herein relate generally to the field of enzymatic conversion, also known as enzymatic hydrolysis, of biomass, (e.g., lignocellulosic material) to obtain monomeric sugars and particularly to maximizing enzyme performance/effectiveness during a liquefaction stage of pretreated biomass.
The production of ethanol from biomass generally entails the following steps: (1) collection and transportation of the biomass to a processing plant; (2) pretreatment of the biomass (pre-hydrolysis) with steam explosion, chemicals (e.g., with or without the addition of an acid or a base), physical means, biological means, and the like; (3) performance of enzymatic hydrolysis using highly specialized enzymes that catalyze the depolymerization of the cellulose into glucose; (4) fermentation of the glucose to ethanol; and (5) separation of the ethanol from the aqueous fermentation broth. Ultimately the separation step removes the last remaining water making a water free ethanol suitable for blending with gasoline.
Several techniques for the pretreatment of biomass material have been explored with the aim of producing substrate that can be more rapidly and efficiently hydrolyzed to yield fermentable sugars. See, e.g., Zheng, Yi, et al., “Overview of Biomass Pretreatment for Cellulosic Ethanol Production,” Int. J. Agric & Biol. Eng ., Vol. 2, No. 3, pp. 51-68 (2009). These approaches have in common the use of conditions and procedures that are designed to increase the surface area to which reactants and enzymes have access. In the case of pretreatment using steam explosion, biomass is fiberized and the cellulose is fractured. To drive the reactions between the enzymes and the pretreated biomass, in particular operating temperature ranges must exist.
Prior to enzymatic hydrolysis of biomass, the biomass may undergo pretreatment involving one or more of the following: acidic condition hydrolysis (with or without the addition of acid), steam explosion, other pretreatment such as ammonium hydrolysis, lime hydrolysis, etc., In particular it may be necessary to cool the pretreated biomass to enhance the enzyme performance during the enzymatic hydrolysis (liquefaction) stage of processing.
For example, biomass, such as lignocellulosic material, may be pretreated to make the sugar based polymers, such as hemi-cellulose and cellulose, accessible to enzymes. After being pretreated, the biomass is processed in an enzymatic hydrolysis reactor vessel(s) where enzymes hydrolyze, e.g., break down, the hemi-cellulose and cellulose polymers to monomers. The pretreated biomass tends to be highly viscous. During enzymatic hydrolysis, the pretreated biomass is liquefied as the polymers of the pretreated biomass are converted to monomers. The monomers, sugars, are further processed into ethanol, butanol or other sugar based products.
Enzymatic hydrolysis of pretreated biomass poses many challenges. These challenges range from the interaction of the enzymes themselves with the biochemical complexity of the pretreated biomass and its derivatives to the physical characteristics of the liquid/fiber, monomeric/oligomeric mixture (collectively referred to as “slurry”) and its rheological features.
Conventional reactors used to perform continuous enzymatic hydrolysis (a process where there are both input and output flows to the process, but the reaction volume is kept constant) required large tanks having expensive and powerful impellers to mix the enzymes into the slurry. Enzymatic hydrolysis or liquefaction of biomass may require several hours, typically more than twelve (12) hours, of mixing in the large tanks. The mixing process reduces the apparent viscosity of the biomass by converting the biomass from a generally solids composition to a liquefied slurry. The pretreated biomass typically starts the mixing process having a semi-solid, mud-like consistency.
Additional conventional enzymatic hydrolysis systems include batch and fed-batch processes. In a batch process, all the components (including pH-controlling substances) are placed in a reactor vessel at the beginning of the enzymatic hydrolysis. During the enzymatic hydrolysis process of the biomass, there is no input into or output from the reactor vessel. An alternative batch process is a fed-batch process. In a fed-batch process (as described in U.S. Patent Publication No. 2010/0255554 A1) nothing is removed from the reactor vessel during the process, but one substrate component is progressively added in order to control the reaction rate by the concentration of the substrate. The substrate is fed continuously into the reactor over the enzymatic hydrolysis period without withdrawing any hydrolysate until the process is complete (as is the case with a batch process).
Biomass is pretreated and subsequently subjected to enzymatic hydrolysis resulting in the conversion to monomeric sugars. The enzymes added to the pretreated biomass typically have a relatively low concentration with respect to the solids content of the pretreated biomass. The pretreated biomass and enzyme mixture tends to be highly viscous as it enters a mixing and enzymatic hydrolysis reactor system. The high apparent viscosity of the mixture has motivated the use of relatively small reactor vessels to reduce the torque needed to mix the mixture while in the reactor vessels. Such a system typically includes one or more enzymatic hydrolysis reactor vessels. The temperature within the enzymatic hydrolysis reactor vessel(s) is important to allow for proper activity of the enzymes. Enzymes typically require a temperature environment of 20° C. (Celsius) to 65° C. Higher temperatures can cause damage to the enzymes, therefore temperature level and temperature level control are important requirements in and around the reactor. The commonly anticipated time span in industrial applications for enzymatic hydrolysis reaction/retention is at least 48 hours, more typically at least 72 hours.
Another challenge in conventional biomass treatment systems is to cool the pretreated biomass to a temperature suited to enzymatic hydrolysis. By way of example, there is a need to cool pretreated biomass from a high temperature, such as 100° C., at the discharge of a pretreatment vessel, to a substantially cooler temperature, such as 20° C. to 65° C., before the biomass enters the reactor vessel(s) for enzymatic hydrolysis.
A conventional enzymatic hydrolysis reactor system is described in U.S. Patent Publication No. 2012/0125549 and discloses adding cold water to the pretreated biomass material that has been subjected to steam explosion in order to reduce the temperature of the pretreated biomass from about 100° C. to the more enzymatically conducive temperature of 40° C. to 50° C. prior to being transferred to an enzymatic hydrolysis reactor vessel. Large volumes of cold water must be added to the hot pretreated biomass to achieve the desired lower temperature, as well as a favorable total solids consistency for successful enzymatic hydrolysis reactions (total solids being defined as soluble/dissolved and insoluble/undissolved solids). This large volume of water undesirably causes a dramatic increase in the volume of liquid being fed to the enzymatic hydrolysis reactor vessel, thereby making consistent control of the volume and temperature to the enzymatic hydrolysis reactor vessel more difficult.
The concentration of the pretreated biomass is indicated by the ratio of pretreated biomass to water. It is advantageous to maintain a high concentration pretreated biomass and a low concentration of water to ensure a high concentration of sugars in the product generated from the enzyme hydrolysis stage. Although adding water to cool pretreated biomass is effective, the addition of water undesirably tends to dilute the pretreated biomass and specifically the sugar solution produced by the enzymatic hydrolyzing of the pretreated biomass.
FIG. 1 shows a process flow diagram showing a conventional continuous system for treating biomass at a rate of fifty tonnes (dry basis) per hour. The biomass is pretreated in a pretreatment vessel 110. In this process the biomass is pretreated at elevated temperatures, such as above 100° C. and in an acidic environment. Other pretreatment processes, including, but not limited to, steam explosion or ammonium hydrolysis or lime hydrolysis, may be used. The pretreated biomass 111, may be discharged from the pretreatment vessel 110 at a rate of 166 cubic meters per hour (m3/h) (for this example the a total solids content of twenty-five percent, 25%, was used), at a total solids content of twenty-five percent (25%) to fifty percent (50%) biomass solids (based on the biomass fed to the pretreatment reactor vessel), and at a temperature of at least 100° C. The total solids content for a biomass treatment process can be twenty-five percent (25%) to fifty percent (50%).
The pretreated biomass 111 is generally too acidic and too hot for enzymatic hydrolysis to occur because the conditions required for pretreatment of biomass are substantially different from the conditions favorable for enzyme hydrolysis. Enzyme hydrolysis usually occurs in an environment having a pH range of about 4 to 6.5 and at temperatures of, for example, about 50° C. to 55° C. Other temperature ranges may be used for enzyme hydrolysis. Yeast based enzyme reactions may occur in a range of, for example, about 28° C. to 40° C., thermophilic bacteria based enzymatic hydrolysis may occur at temperatures as high as about 80° C., and mesophilic bacteria based enzymatic hydrolysis may occur at temperatures between about 20° C. and about 45° C. A mixing stage 112, such as a mixing vessel, is used to adjust the pH and cool the pretreated biomass 111. An appropriate base 114 (a base is used in this example as the pretreatment conditions were acidic), such as ammonia, lime or other earth metal based hydroxide or carbonate, is added during the mixing stage 112 to adjust the pH of the pretreated biomass 111 to a level suitable for enzymatic hydrolysis.
A cooling liquid is added via conduit 116 in the mixing stage 112 to the pretreated biomass 111. The cooling liquid has typically been water, stillage or other suitable liquid from the mill. It has been proposed to use fully enzyme hydrolyzed biomass product 121. Typically after both pretreatment and enzymatic hydrolysis the biomass has a monomeric sugars yield of at least thirty percent (30%), and is discharged from a large enzymatic hydrolysis reactor vessel 118. The fully enzyme hydrolyzed biomass 121 may be split, with a portion via conduit 123 being pumped via pump 120 through a cooling stage 122 and to the mixing stage 112 via conduit 116.
In the mixing stage 112, batch mode mixing vessels are used to convert the pretreated biomass 111 to conditions suitable for enzymatic hydrolysis. Batch mode generally involves several smaller mixing vessels that feed a larger downstream vessel (not shown), such as a digester or other reactor vessel. Batch processing increases the volume needed in the mixing stage 112 vessels to accommodate the filling and emptying portions of each cycle for the mixing stage 112 vessel.
Enzymatic hydrolysis of pretreated biomass typically requires many hours to complete. The retention period in a large enzymatic hydrolysis reactor vessel 118 or assembly of vessels, for example, may be 24 to 72 hours or longer. The fully enzyme hydrolyzed biomass product 121 is discharged from the enzymatic hydrolysis reactor vessel(s) 118 and used as a manufactured monomeric sugar solution 119. Due to the long enzyme hydrolysis process periods, the volume of the large enzymatic hydrolysis reactor vessel(s) 118, e.g., tanks, are large for example could be as large as 37,000 m3 or more. The high volume capacity, large size, required for the enzymatic hydrolysis vessels of the conventional continuous system results in limiting the biomass material to be processed.
A conventional large enzymatic hydrolysis reactor vessel 118 for enzyme hydrolysis tends to be much larger than a corresponding pretreatment vessel 110 used to pretreat the biomass. The pretreatment vessel 110 is much smaller because the pretreatment periods are typically much shorter than the process periods for enzyme hydrolysis.
As shown in FIG. 1 it is known to use, previously fully enzyme hydrolyzed biomass 121 as a cooling liquid to substitute in whole or part for water 130 to maintain a high concentration of pretreated biomass 111 in the mixing stage 112. The fully enzyme hydrolyzed biomass product 121 has been fully hydrolyzed (both from pretreatment processes and enzymatic hydrolysis) may be used as a manufactured monomeric sugar solution 119. The use of the fully enzyme hydrolyzed biomass product 121 as a cooling liquid avoids excessive dilution of adding cooling water 130. The use of fully enzyme hydrolyzed biomass product 121 as cooling liquid increases the needed volume of the large enzymatic hydrolysis reactor vessel(s) 118, e.g., tanks. Large enzymatic hydrolysis reactor vessel(s) 118 are needed to accommodate the long residence times, e.g., 24 to 72 hours or longer, needed for proper conversion of the pretreated biomass 111 from the pretreatment vessel 110, composed primarily of polymeric cellulose and hemi-cellulose to monomeric sugar solutions 119.
Other means for cooling pretreated biomass 111 (e.g., cooling gases or a cooling jacketed conveyor system) are known but may not be suitable for all process flow systems. Cooling gases, for example, often do not readily penetrate pretreated biomass 111 due to the fine particulate sizes of the pretreated biomass 111. Indirect heat exchangers are also often not suitable because the highly viscous pretreated biomass 111, e.g., a mud like consistency, does not readily flow through the passages of the heat exchanger. A cooling jacketed conveyor system between the pretreatment vessel 110 and the large enzymatic hydrolysis reactor vessels 118 may be used to cool pretreated biomass 111. However, the amount of heat transfer achieved in cooling jacketed conveyor systems may not be sufficient to cool the pretreated biomass 111 to the temperatures appropriate for enzymatic hydrolysis due to small area-to-volume ratios in the large diameter tube of the cooling jacketed conveyor systems and poor contact between the pretreated biomass 111 and the surfaces of the tube.
There is a long felt need for temperature and volume control of the pretreated biomass material to the reactor while reducing the need for the addition of fresh, cooling liquid (e.g., water) to the hot pretreated biomass from the appropriate pretreatment process.